ABB AC700 PLC as Modbus RTU Master & 4-5 RTU slaves Communication Problems
I have 3 systems with ABB AC700F PLCs as Modbus RTU Masters & 4 or 5 RTU Slaves.
Each system works separately (PLC are physically disconnected from each other)
My problem is a communication failure between Master and slaves after 2 or 3 or 4 weeks of perfect working. The only one decision which solving the porblem after the communication failure is a cold restart of the master PLC. Physilally RS-485 link is perfect, but those failures occur in any case of program examples that I used before for three years of experiments.
Has anyone such Modbus RTU communication problems?
Technical characteristics of my communication task:
- Modbus RTU Master - ABB PM783 Controller - 1 unit;
- Modbus RTU Slaves - ABB P820 Panel - 2 units; Danfoss FC-102 - 3 units;
- Baudrate - 38400 bits/sec;
- Physical interface - RS485;
- Cable - shielded twisted pair for RS485 networks;
- Network structure - "chain with PLC inside" according to any RS485 network sructure and described in ABB manuals;
- Application - Water Heating Plant;
- Communication Timeouts (configured in Modbus function blocks): ABB P820 Panels = 100msec; Danfoss FC-102 = 50msec;
- Actual Response Time (this value appear on the "RSP" pin of the Modbus R/W FB): for ABB P820 Panels = 60-70 msec; for Danfoss FC-102 = 25-30 msec;
- Number of retries: ABB P820 Panels = 1/1 (for read/write); Danfoss FC-102 = 0/1 (for read/write);
- Duration of PLC Program cycle >= 300 msec;
- Maximum quantity of Modbus send/receive operations during 1 program cycle - not more than 2 ("AUTOREQUEST" mode in not used because of its misoperability discovered at the start of this job, i.e. each PLC cycle only 1 or two of Modbus R/W function blocks receive "TRUE" on its "REQ" input.);
- Average Timeouts Quality (TO = (Timeouts / Transmit Count)*100 = 1.5 %). Best is TO = 0,01 %!!!
- Process Image Size (Configurable in Task Header) - 4 or 32 kB - No matter!
As usual system runs properly, but sometimes (once in 2 or 3 or 4 weeks or longer) Receive count stops to increment, but Transmit count and Timeouts continue to increase (PLC sends communication requests, but all slaves cannot receive any request, so all Master requests become unreceived by the slaves). As I said - the only working decision to solve this problem is a cold restart of the master PLC (by means of power supply reset or remotely by means of reload of the whole station in Control Builder). Screenshots that decribe the problem and after restart - see in attached files (pls take a deep look at the communication statistics before and after!).
So, please, help me to solve this problem!!!
Our local ABB representative (ABB - Ukraine) cannot help!!!
Insert the URL that your highlighted text will link to.
Copy the video's "embed" tag into Embedded Video below.